Fix for Rails 6.x where the multipart form enctype was not being applied

[westonganger]

Solves #20

In Rails 6.x, form_for ..., multipart: true was not correctly applying the enctype="multipart/form-data" to the HTML form tag

Turns out that in Rails 6.x and prior you are supposed to either use:

• form_for ..., html: {multipart: true}
• OR
• If the form uses the file_field method then the enctype is automatically added

Both of the potential fixes for Rails 6.x, also work in Rails 7.0 and above